Umbraco.Elasticsearch is only available as a NuGet package and can be installed into an existing solution using:

Installing from NuGet

Install-Package Umbraco.Elasticsearch

This will install both the UI package (Umbraco.Elasticsearch) and the core library (Umbraco.Elasticsearch.Core).

Umbraco.Elasticsearch takes a dependencies on the Full UmbracoCMS package, while Umbraco.Elasticsearch.Core only depends on the UmbracoCMS.Core package. This means if you have a solution with multiple projects you can take a dependency on the Umbraco.Elasticsearch.Core package in all your non-web projects and avoid pulling in unnecessary plugin files.

Once installed you will have to configure the integration

Installing from the Umbraco Package Repository

Umbraco.Elasticsearch is not available as an Umbraco package.